In the past decade, the science of genetic engineering has developed rapidly. A variety of processes are known for inserting a heterologous gene into bacteria, whereby the bacteria become capable of efficient expression of the inserted genes. Such processes normally involve the use of plasmids which may be cleaved at one or more selected cleavage sites by restriction endonucleases, discussed below. Typically, a gene of interest is obtained by cleaving one piece of DNA and the resulting DNA fragment is mixed with a fragment obtained by cleaving a vector such as a plasmid. The different strands of DNA are then connected ("ligated") to each other to form a reconstituted plasmid. See, for example, U.S. Pat. Nos. 4,237,224 (Cohen and Boyer, 1980); 4,264,731 (Shine, 1981); 4,273,875 (Manis, 1981); 4,322,499 (Baxter et al, 1982), and 4,336,336 (Silhavy et al, 1982). A variety of other reference works are also available. Some of these works describe the natural processes whereby DNA is transcribed into messenger (mRNA) and mRNA is translated into protein; see, e.g., Stryer, 1981 (note: all references cited herein, other than patents, are listed with citations after the Examples); Lehninger, 1975. Other works describe methods and products of genetic manipulation; see, e.g., Maniatis et al, 1982; Setlow and Hollaender, 1979.
Most of the genetic engineering work performed to date involves the insertion of genes into various types of cells primarily bacteria such as E. coli, various other types of microorganisms such as yeast, and mammalian cells. However, many of the techniques and substances used for genetic engineering of animal cells and microorganisms are not directly applicable to genetic engineering involving plants.
As used herein, the term "plant" refers to a multicellular differentiated organism that is capable of photosynthesis, such as angiosperms and multicellular algae. This does not include microorganisms, such as bacteria, yeast, and fungi. However, the term "plant cells" includes any cell derived from a plant; this includes undifferentiated tissue such as callus or crown gall tumor, as well as plant seeds, propagules, pollen, and plant embryos.
A variety of plant genes have been isolated, some of which have been published and/or are publicly available. Such genes include the soybean actin gene (Shah et al, 1982), corn zein (Pederson et al, 1982) soybean leghemoglobin (Hyldig-Nielsen et al, 1982), and soybean storage proteins (Fischer and Goldberg, 1982).